Observation of VLF Signal Pulsations Induced by Solar Flare X‐ray Emissions on 4 March 2023

Abstract

We present a case study of the D‐region ionospheric response to a sequence of solar flares, comprising one M‐class and four C‐class flares on March 4, 2023. Very Low Frequency (VLF) signals from the VTX (18.2 kHz) and NWC (19.8 kHz) transmitters were recorded at the low‐latitude station Dehradun (DDN, 30.31°N, 78.03°E), India. The flares originated from different active regions of the Sun and produced pulsations in the integrated solar X‐ray flux, which were accompanied by pulse‐like variations in the VLF signal amplitude along the VTX‐DDN propagation path, whereas no clear pulsations were observed along the NWC‐DDN path. The temporal structure of the combined X‐ray flux, rather than the spatial origin of the individual flares, appears to be the primary driver of the observed VLF response, consistent with the reported insensitivity of the D‐region response to flare location on the solar disk (Hayes et al., 2021, https://doi.org/10.1007/s11207‐021‐01898‐y ). While Hayes et al. (2017), https://doi.org/10.1002/2017ja024647 reported comparable pulse‐like VLF variations associated with flares originating from a single active region, our results suggest that a sequence of flares from multiple active regions can also produce similar pulsations in the D‐region ionosphere. These observations demonstrate the sensitivity of the D‐region to temporal variations in solar soft X‐ray flux and highlight the coupling between solar activity and the Earth's lower ionosphere. LWPC modelling indicates a maximum decrease of approximately 2 km in the D‐region reference height and an increase of 0.11 km ⁻¹ in the sharpness factor during the X‐ray event of 4 March 2023 along the VTX‐DDN propagation path.